Discharge valves can become leaky because of too little valve play or because of damaged sealing surfaces. As a consequence, the following occurs: exhaust gas from previous work strokes enters the cylinder when the fresh gas charge is inducted for the next work stroke, the quality of the next combustion is reduced and thereby negatively affects the exhaust gas composition and the even running of the internal combustion engine. The catalytic converter of the engine can be overheated because of secondary reactions when the quality of the combustion is greatly reduced.
Methods for detecting combustion misfires are known, for example, from U.S. Pat. No. 5,255,560. Combustion misfires or uneven running fluctuations can also occur because of defects of the ignition equipment or a defect in the formation of the mixture can be caused, for example, by defective injection valves. For this reason, leaky exhaust valves cannot be clearly identified as the cause of a fault with the known methods. Up until now, leaky exhaust valves can be identified only with relatively great difficulty and service station means, for example, by disassembling the cylinder head.